The Ninth Circle (Tour of the Merrimack #5)

Fifth in the hard-hitting military science-fiction series. On the distant world of Zoe, an expedition finds DNA-based life. When alien invaders are also discovered, Glenn Hamilton calls on the U.S.S. Merrimack for help.

Long Synopsis:

When peace exists between the Terran and Palatine Empires, the troops aboard the U.S.S. Merrimack are always on edge, waiting for the Roman forces to violate the treaties in a sneak attack on some unprotected corner of the galaxy. But as far as anyone can tell all is quiet in the universe.
Admiral John Farragut no longer walks the decks of the space battleship he commanded for so many years, his time now spent on Earth with his wife and child.
Captain Calli Carmel has taken Farragut's place on the ship they've both called home for so long, and she commands the Mack firmly and fairly.
Others, too, are absent from their posts. Lieutenant Glenn Hamilton is on a well-deserved leave, traveling with her husband, Dr. Patrick Hamilton, the ship's xenolinguist. They're aboard a League of Earth Nations ship on a scientific expedition to an Earthlike world named Zoe, located at the back of beyond at the edge of the galaxy. Glenn is resigned to a quiet and boring run--a chance for Patrick to do what he does best while Glenn takes a back seat and tries not to upset the researchers, who can't abide any military presence. But when their vessel is attacked upon arrival, Glenn is forced to seize control, managing to crash-land the ship with no loss of life. Then--thanks in part to Patrick and Glenn--the expedition discovers that life on Zoe is DNA based, an extremely rare occurrence. So when alien invaders are also discovered on the planet, and they begin wreaking havoc on both the humans and native life-forms, it is only natural for Glenn Hamilton to call on the U.S.S. Merrimack for help.
Meanwhile, on the Roman world of Phoenix, the hazing of a new recruit has gone horribly wrong. His untimely death leads the remaining members of his unit into permanent exile. Condemned to the hell of being considered dead to all they hold dear, they steal a cutting edge spaceship and, dubbing themselves the damned of the Ninth Circle, become pirates. Their leader is a man called Nox--formerly known as John Knox Farragut--younger brother to the admiral.
Inevitably, both the Ninth Circle and the Palatine Empire find their way to Zoe. And suddenly all sides are on a collision course to determine the fate of this uniquely valuable planet....